Need to understand how to show reflected light well with cycles
This is pretty much the worst-case scenario for backwards path tracing.

On contact with the diffuse plane the ray could go in literally any direction, and only a tiny fraction of those directions will hit the glossy object at the right place and angle to rebound into the smallish light source.



To see why increasing the roughness of the glossy material helps, look at it from the point of view the ray right after hitting the diffuse plane:



As you can see, the area which the ray can hit and still find the lamp is much larger even with only .1 roughness. In addition, MIS probably can do a bit more to direct samples to the lamp when there is some roughness and the ray doesn't need to bounce at an exact angle (as is the case with specular reflection).

Note that with enough samples it does eventually converge (in this sense these aren't fireflies so much as noise). Here's a render with 10000 samples: